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Fibrinogen adsorption to human erythrocytes Janzen, Johan


Fibrinogen adsorption to human erythrocytes has been implicated as the mechanism responsible for the reversible aggregation of red cells. The object of this thesis was to measure the binding of fibrinogen to human erythrocytes and to compare the binding with aggregation behaviour. The binding of isolated ¹²⁵I-fibrinogen to red cells was estimated from the amount of radiolabel removed from solution upon addition of cells, from the amount of radiolabel retained in cell pellets produced by ultracentrifugation and corrected for intracellular radiolabel, and from radiolabel retained by cells after exhaustive washing in protein-free buffer. Binding of human albumin to red cells was determined by the same methods. The free energy of formation of red cell/red cell contacts due to isolated fibrinogen was determined by the method of Evans and Buxbaum (Biophys. J. 3̲4̲, 1-12, 1981). The solution depletion method required corrections for dilution effects and was not sensitive enough for detailed determination of binding of these proteins to red cells. Binding estimated from cell pellets and corrected for intracellular radiolabel increased linearily with protein concentration to 12 mg/mL for fibrinogen and 50 mg/mL for albumin. The binding at 1 mg/mL was between 50 and 250 molecules per cell for fibrinogen and between 1000 and 1400 molecules per cell for albumin. Binding determined from wash analyses was similar to pellet binding corrected for free radiolabel. The surface affinity of red cells for red cell beads was determined to be 1.8 ± 0.5 x 10⁻³ erg/cm² (mean ± SEM, n = 4) and 2.8 ± 0.8 x 10⁻³ erg/cm² (mean ± SEM, n = 28) at 4.4 and 8.8 mg/ml fibrinogen respectively. Adhesion between red cells and red cell beads occurred at 2.2 mg/mL fibrinogen, but was too weak to allow estimation of the surface affinity. The surface affinities and binding estimates allow calculation of the mean binding energy for fibrinogen to the red cell surface. The surface affinity per molecule, assuming 250 molecules per cell at 1 mg/mL, was 30 ± 10 and 24 ± 7 kilocalories/mole at 4.4 and 8.8 mg/mL respectively. The difference was not significant. This result poses a problem as binding energies of this order suggest that the adhesion would be irreversible. If fibrinogen binding were 1500 molecules per cell at 1 mg/mL a binding energy of 4 ± 1 kilocalories/mole would be implied. These results are interpreted as indicating that fibrinogen bound to red cells may be displaced during ultracentrifugation.

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